The present invention relates to a digital magnetic recording and reproducing apparatus for reproducing data recorded on a recording medium in a digital fashion by a reproducing head of a magnetoresistive type (MR head), and more particularly concerns a magnetic recording and reproducing apparatus preferably for reproducing data recorded at a high density and for transferring the data at a high rate. Also, it relates to a magnetic recording and reproducing apparatus for reproducing data recorded on a magnetic recording medium using a reproducing magnetic head made of a magnetoresistive element (MR element), and more particularly concerns a magnetic recording and reproducing apparatus for high-density recording and for a high-rate transfering of data with compensation for non-linear distortion proper to the MR element and further particularly concerns a magnetic recording and reproducing apparatus having a reproduced signal processing circuit preferably to prevent the MR element from damage due to its intermittent contact with the magnetic recording medium or due to an overcurrent upon its floating above an extremely narrow gap and to suppress possible electromigration.
A magnetic head of inductive type has been conventionally used for recording on or reproducing data from a magnetic recording medium. However, its requirements in view of recording, for example, the pole-tip length and the length of a gap between the pole-tips, do not always coincide with the requirements in view of reproduction.
To increase the recording density, for example, a coercive force of the recording medium has to be increased. For recording on the recording medium having a high coercive force, the recording magnetic field from head has to be made high. Accordingly a higher recording magnetic field requires greater pole-tip and gap length. In view of highly sensitive, highly resoluble reproduction, on the other hand, it is rather preferable to make the pole-tip length and gap length small. In such requirements, the recording head should be separated from the reproducing head.
On the other hand, if the inductive type reproducing head has a coil having an increased number of turns to make the reproduced voltage high, head noises become high , and increased head pole-tip length lowers the reproduction efficiency. These factors also undesirably lower the signal-to-noise ratio, resulting in an increase of jitter. In order to solve such problems, it is effective to use a magnetoresistive element for the reproducing head as it reduces the head noises.
The magnetoresistive element makes use of its physical property that its electrical resistance is changed according to a sensed intensity of a magnetic field so that it can obtain the intensity of the magnetic field by detecting the change of the electrical resistance.
In the prior art, in order to detect the intensity of the magnetic field, or the electrical resistance, a constant dc current called the sense current is made to flow to have a voltage drop across the magnetoresistive device. The highest voltage drop and the lowest one correspond to reversal positions of magnetization, which are detected as data of `1` and `0`. In the prior art is used a level detection method for discriminating `1` and `0` depending on the voltage level. Change of the electrical resistance of the magnetoresistive element to the magnetic field applied thereto is non-linear as shown in FIG. 4.
The electrical resistance differs in its rate of change in the input positive magnetic field and the negative one. The positive polarity and negative polarity of change in the electrical resistance thus have different amplitudes. In the prior art level detection method is controlled a biased magnetic field so that the upper and lower half amplitudes should be made equal, as in the Japanese Patent Application Laid-Open No. 03-12005. For higher recording density and higher data transfer rate, however, the data discriminating window has to be made narrower in its period of time. It thus is difficult to detect the level.
For the reason, a phase discrimination method has been employed as a method which is easier to implement than the level detection method for detecting the reversal of magnetization with little error.
There have been many prior disclosures about the MR element and reproducing circuit system having it used therewith to reproduce the magnetic data recorded on the recording medium. The MR element is an element the electrical resistance of which corresponds to a vector value of the input magnetic field. The MR element has a biased magnetic field applied thereto if the magnetic data are reproduced from the recording medium. Resistance of the MR element is changed as magnetic flux is made to flow thereinto from the magnetic recording medium. The change of resistance is converted to change of voltage across the MR element by a sense current. A succeeding reproducing circuit amplifies the voltage across the MR element to reproduce the magnetic data recorded on the recording medium.
However , the prior art reproduction of the recorded data with use of the MR element has the disadvantage that the reproduced waveform by the MR element involves non-linear distortion as the change of the resistance of the MR element to the flowing magnetic device is properly non-linear as described above.
Therefore, two MR element have been conventionally combined together differentially to take out the differential signals only. The differentially combined differential signals solve the problem of non-linear distortion as their non-linear distortions cancel out each other if any.
Also, the prior art MR element has the disadvantage that voltage drop is caused by the sense current as it is a kind of resistor. The MR element therefore has a potential difference from a reference potential. Since the recording medium is ordinarily set at the reference potential, a potential difference is caused between the MR element and the recording medium. If the MR element is brought contact with or extremely close to the recording medium, overcurrent will flow between the MR element and the recording medium, resulting in damage of the MR element. To solve that problem, as an example, there has been proposed a technique in the Japanese Patent Application Laid-Open No. 2-94103. In the technique, the MR element has a sensor placed in parallel therewith to detect the potential thereof. On the basis of the signal output of the sensor, a bias current for setting the potential of the MR element is controlled in a negative feedback way. After the feedback, the bias current and the sense current for the MR element are controlled to be equal. Since magnitude of the sense current cannot be controlled, however, the technique cannot resolve the problem of non-linear distortion.
In particular, the prior art phase discrimination method for the inductive type head is used properly for the high density recording and high rate data transfer, the peaks of the reproduced waveform are dispersed in the sharpness even with use of the biased magnetic field method disclosed in the Japanese Patent Application Laid-Open No. 03-12005. The peaks thus have different gradients at the zero-level crossing point when differentiated.
The phase discrimination method is a method that a zero cross pulse is generated at the zero-level crossing point of the differential waveform reproduced by the reproducing head. The data of `1` and `0` can be discriminated depending on whether the pulse is present or not in the data discriminating window. In the method, such a virtually constant amount of noises as the circuit noise and head noise are superimposed on the reproduced waveform. The dispersion of jittered zero cross pulses becomes wider at a slow gradient of the differential waveform. This may cause a discrimination error so that it can prevent the high recording density and high rate data transfer.
In order to solve the problem of non-linear distortion, as described above, the two MR elements have been conventionally combined together differentially to take out the differential signals only. The track width is made narrow with the high density recording of the magnetic recording apparatus. For the reason, reproduction with only one MR element is needed. Since the resistance change of the MR element to the magnitude of flowing magnetic field proper to the MR element is non-linear, the peak of the reproduced waveform involves the non-linear distortion. If a reproduced signal process (differentiation detection) is made in the state of such a reproduced waveform, reproduction error may be caused on the peak side of greater distortion.
Further, since the MR element is a kind of resistor, voltage drop is caused by the sense current, as described above, the MR element has the potential difference to the reference potential. The magnetic recording medium, on the other hand, is ordinarily set at the reference voltage. The potential difference is caused between the MR element and the recording medium. If the MR element is brought contact with or extremely close to the recording medium, overcurrent will flow between the MR element and the recording medium, resulting in damage of the MR element.